Redox flow batteries (RFB) have attracted significant attention as an important energy storage system. Advantages of RFB systems include separation of energy capacity and power output, long service life, relative safety, and ease of manufacturing. As a result, RFB systems compose a leading consideration for applications such as large-scale energy storage and integral components in electrical power infrastructure.
However, both technical and economic barriers have limited commercial uptake of RFBs. For example, long-term operation stability and high cost have limited the technology from broader market penetration. In many instances, these barriers are associated with the membrane that provides physical separation between the positive and negative electrolytes to prevent their cross-mixing while allowing passage of charge carriers to complete the circuit.
In many mature RFB systems, perfluorinated ion exchange materials are used as the membrane. However, these materials are not preferable at least because they are extremely expensive and can account for 40% of the total cost of a RFB stack, which comprises a plurality of RFB cells, or repeat units. Furthermore, in all-vanadium RFBs, for example, the use of these materials (e.g., NAFION®) can result in capacity decay due to the different transfer rates of the four vanadium ions and the asymmetrical valence state of vanadium ions caused by subsequent self-discharge reactions between the transferred and native vanadium ions.
One alternative to a perfluorinated ion exchange membrane is a microporous separator. However, it is unlikely that typical microporous separators can withstand the harsh chemical environment of certain RFBs. Accordingly, a need for improved RFB separators exists to reduce cost, improve performance, and/or enable additional property-enhancing developments.